The present disclosure relates to a developing device for developing electrostatic latent images with toner, as well as to an image forming apparatus including the developing device.
Among such image forming apparatuses as multifunction peripherals, copiers, printers and facsimiles are those which use a developer including magnetic carrier and toner (so-called two-component developer) to fulfill printing. In developing process, only toner is consumed out of the magnetic carrier and the toner. Therefore, when a toner density in the developer has decreased in printing, there is a need for adding (supplying) toner to the developer. In this connection, a sensor using an LC oscillation circuit may be used for measurement of the toner density. A signal outputted by the LC oscillation circuit varies in frequency in response to the toner density (magnetic carrier density) in the developer. Based on such frequency changes, a toner density in the developer is detected. A known example of developing devices including an LC oscillation circuit is as follows.
As an example, there is known a developing device which includes a developer carrier, a developer housing tank, an LC oscillation circuit, and a toner density detector placed at a position where the inductance of a coil constituting the LC oscillation circuit is affected by toner density of the developer in the developer housing tank, in which arrangement the coil is made up from a scrolled print pattern which is formed on a flat plate and which has a print pattern-free hollow region at a central portion of the scroll. This structure is intended to provide a nearly flat distribution of detection sensitivities in the print-pattern central portion so that the detection result is less affected by variations of the developer quantity within the detection region.
As described above, in image forming apparatuses with use of a developer containing magnetic carrier and toner, a sensor including a coil may be used to detect the toner density (ratio of toner in the developer). In other words, the sensor uses a coil to detect increases of the ratio of magnetic carrier (magnetic-field variations) in the developer due to toner consumption.
With regard to a sensor using a coil, the more the distance between the developer and the coil increases, the more the variation in inductance of the coil relative to the variation in toner density decreases, causing the variation in sensor output (variation extent of frequency) to decrease. That is, the more the distance between the developer and the coil increases, the more difficult it becomes to detect the toner density accurately.
Conventionally, the sensor coil is placed outside the housing of the developing device. Inside the developing device, various members such as a rotator are provided. The developing device is required to have a certain level of strength. Therefore, the housing thickness cannot be made thinner than a certain level. The housing thickness causes the sensor coil to be distanced from the developer. Conventionally, the toner density detected by the sensor using a coil may involve large errors, hence a problem in terms of precision.